United States Patent (19)

Transcription

1 United States Patent (19) Lowy et al. 54 NK RETURNSYSTEM FOR A MULTJET HNKJET PRINTER 75 Inventors: Paul Lowy, Peekskill; Stanley Arthur Manning, Yorktown Heights; Karl Friedrich Stroms, Wappingers Falls, all of N.Y. 73) Assignee: International Business Machines Corporation, Armonk, N.Y. 22 Filed: July 28, 1972 (21) Appl. No.: 275, U.S. C /1, 346/75, 346/140 51) Int. Cl.... G01d 15/18 58) Field of Search /75, 140 (56) References Cited UNITED STATES PATENTS 3,404,221 10/1968 Loughren /75 X FOREIGN PATENTS OR APPLICATIONS 1,123, 188 8/1968 Great Britain /75 (11) (45) Mar. 19, 1974 Primary Examiner-Joseph W. Hartary Attorney, Agent, or Firn-Sughrue, Rothwell, Mion, Zinn & Macpeak 57 ABSTRACT A multi-jet, inkjet printer having deflection plates at a positive or negative potential with respect to ground which includes catch chambers for returning unused ink to an ink reservoir. Non-information bearing ink droplets are passed uncharged to a catch chamber and from there through a return conduit system to the ink reservoir. To help direct the ink droplets to the catch chambers, the catch chambers are mechanically bi ased toward the trajectory of the uncharged droplets by placing them at a slight angle with respect to the trajectory. To prevent short-circuiting of the deflec tion plates to ground potential, the return conduit sys tem includes denebulization chambers which convert the returning ink stream into large drops thus develop ing a high resistance path between the deflection plates and the reservoir ink at ground potential. 10 Claims, 1 Drawing Figure

2 PATENTED MAR

3 INK RETURNSYSTEM FOR A MULTJET INKJET PRINTER BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is in the field of the inkjet printers and more particularly in ink return systems for such print e.s. 2. Description of the Prior Art Inkjet printers have created much interest in the area of high speed printing. For example, in computer sys tems, it is often desirable to print out information at speeds of up to 4,000 characters per second. Inkjet printers have this capability and may be used in con junction with or in place of conventional CRT type dis play units. In inkjet printing, one or more inkjet producing noz zles, connected to an ink reservoir through a pressure pump, receive fluid ink under pressure and eject the ink in fine continuous sprays, each of which comprises a string of droplets. The continuous ink spray inher ently breaks up into very small droplets. In some sys tems, the rate and size of these droplets are controlled by vibrating the nozzles. The particular vibration fre quency is controlled to control the size and spacing of the droplets. The stream breaks up into droplets as it passes through a charging electrode. The potential on the charging electrode is varied in accordance with the information to be printed. As the droplets pass through the electrode, a charge is transferred to the individual droplets, this charge being a function of the potential applied to the charging electrode to thereby produce information bearing droplets. Between the charging electrode and the moving printing medium upon which the information bearing droplets impinge, there is gen erated a fixed electrostatic field which changes the tra jectory of the droplets passing therethrough, in accor dance with the charge thereon, whereby the droplets are directed to selected points on the moving printing medium. An example of such a printer is illustrated in the article by R.L. Gamblin et al, Electrostatic Ink De flection Bar Code, Printer, IBM Technical Disclosure Bulletin, Volume 1 1, No. 9, May 1969, pages Since the flow of liquid ink is continuous, a catch basin must be used to gather unused ink. The unused ink is that ink which does not carry an information in dicative charge. For the purpose of describing this in vention, ink jet printers will be classified into two groups, those which position the catch basin between the deflection plates and the printing medium and those in which the catch basin is integral with the de flection plates. In the former type system, exemplified by U.S. Pat. No. 3,484,793, to G.A. W. Weigl, issued Dec. 16, 1969, the droplet trajectory length is long, creating problems of aerodynamic instability of the droplets. A technique for improving droplet stability is to shorten the trajectory path. A convenient means for ac complishing this is to make the catch basin integral with the deflection plates thereby permitting the print ing medium to be placed close to the deflection plates. Such a system is described in U.S. Pat. No. 3,512,173 to D.E. Damouth, issued May 12, In the Damouth system a pair of deflection plates are asso ciated with each nozzle, with one of the plates being placed at ground potential. The grounded deflection plate has associated therewith an intercepting plate which together with the deflection plate forms a return channel for the unused ink. A high voltage source is coupled to the other deflection plate to provide a po tential difference in the area of 3000 volts between the two plates. To direct non-information bearing ink drop lets into the return channel, a uniform bias charge is ap plied thereto. Thus each droplet, whether it carries in formation or not, is provided with a charge. The prob lem with this technique is that the charge on the unused droplets affects the charge on the information bearing droplets. Further, by requiring one of the deflection plates to be at ground potential, a separate pair of de flection plates must be utilized with each nozzle. This requirement results from manufacturing difficulties as sociated with the positioning of the plurality of nozzles relatively close together. Still further, very high poten tial sources must be used with the system. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved multi-jet, inkjet printing apparatus which in cludes a novel ink return system of the type which in cludes deflection plates formed with catch chambers integral therewith. To accomplish the above objective, there is provided a multi-jet, inkjet printing apparatus wherein fluid ink stored in an ink reservoir is supplied under pressure to a plurality of nozzles to produce a continuous ink spray from each nozzle. A moving printing medium is posi tioned to receive the ink. Associated with each nozzle is a charging electrode and a pair of deflection plates. The ink ejected from the nozzles breaks up into small droplets as these droplets pass through a charging elec trode. The charging electrodes are caused to assume a potential indicative of the information to be printed. When no printing is to take place, the charging elec trode is placed at ground potential so that no charge is imparted to the droplets passing therethrough. As droplets pass through a charging electrode at a poten tial indicative of information to be printed, a charge is developed on the droplet, the charge being a function of the potential applied to the charging electrode. The droplets pass through an electrostatic field created by a pair of deflection plates whereby charged droplets impinge upon the printed medium at selected locations. Non-information bearing droplets remain uncharged so that they do not interfere with the charge on the in formation carrying droplets. The deflection plates are formed with a hollow portion acting as a catch chamber for the uncharged, non-information bearing droplets. To assist in the catching of uncharged droplets, the de flection plates are disposed at a small angle with re spect to the trajectory of these droplets. Adjacent deflection plates are given respectively a positive and negative potential with respect to ground and adjacent inkjet nozzles share a common deflection plate thus reducing the total number of plates needed. For example, in a system utilizing 60 nozzles and 60 charging electrodes, only 61 deflection plates are re quired. In that none of the deflection plates is at ground potential, the magnitude of the potential applied to any one plate can be greatly reduced thus permitting the use of less expensive, lower potential sources. That is, instead of using for example a 3000 volt source coupled to a deflection plate, the other plate being at ground

4 3 potential, one plate is coupled to a volt source while the other plate is coupled to a volt source. An opening is provided at the end of each deflection plate to receive fluid collected by the catch chamber. The opening is coupled to a return conduit system for carrying the unused ink back to the ink reservoir. How ever, as the ink enters a catch chamber, it assumes a charge which corresponds to the potential on the plate. So that the returning fluid does not provide a low resis tance path between the reservoir ink at ground poten tial, and the deflection plates, thereby presenting a short-circuit path from the deflection plate to ground, denebulization chambers are provided within the re turn conduit system. As used herein denebulization re fers to the breaking up of a fluid stream into relatively large drops. These chambers convert the returning sub stantially continuous flow of ink into relatively large drops which present a high impedance to current seek ing to flow between the deflection plates and the ink reservoir. BRIEF DESCRIPTION OF THE DRAWING The FIGURE illustrates the improved non-shorting ink return system of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the FIGURE, information received from information sources 5 is to be printed on a printing medium 3 by selectively deflecting inking droplets pro duced by a plurality of nozzles 6. To this end, ink 2, stored in ink reservoir 4 is supplied in a conventional manner to the nozzles 6. Ink pump 8 is provided to in troduce the ink into the nozzles 6 under pressure. The ink forced into the nozzles 6 is then ejected therefrom toward the printing medium 3. Due to the natural insta bility of the liquid stream 10, the ink inherently breaks up into small droplets 12 a short distance from the noz zle. In the vicinity of the ink break-up there is posi tioned a charging electrode 4 coupled to an informa tion signal source 15. The information signal source provides a potential on its associated charging elec trode indicative of the information which is to be printed. In its simplest form, the information signal source may be a ramp generator selectively turned on or off. As the ink droplets 12 pass through a charging electrode 14, they assume a charge which is a function of the potential on the charging electrode. The ink stream 10 is continuous and when no information is to be printed on document 3, the potential on a charging electrode 14 is at ground and thus uncharged ink drop lets emerge from the charging electrode. To collect the uncharged ink droplets, each deflection plate 7 is pro vided with a catch chamber 16 for receiving the un charged droplets. Each nozzle 6 is mechanically aimed at a catch chamber 16. Two deflection plates are asso ciated with each nozzle and create a fixed electrostatic field which alters the trajectory of charged droplets. One of these plates is at a potential positive with re spect to ground while the other is at a potential nega tive with respect to ground. To aid in the catching pro cess, the deflection plates are positioned at a slight angle with respect to the trajectory of the droplets. As illustrated in the FIGURE, each of the deflection plates 17 is positioned at an angle of approximately 3 with respect to the trajectory of uncharged droplets. 4. ink collected by the catch chambers 16 flows through openings 18 into either the return conduit 20 or 22 de pending upon whether the fluid has emerged from a positive or negative potential deflection plate. The con 5 duits may be formed of insulating material for safety reasons. However, as uncharged droplets enter a catch chamber, they assume a charge corresponding to the potential on that plate 17. Ink flows out of a deflection plate through return conduit 20 or 22 in the form of a 10 substantially continuous flow which if it were con nected would present a low resistance path between the deflection plate and the reservoir ink at ground poten tial. If such a low resistance path occurs, shorting out the deflection plates might result. 15 To prevent this, there are provided denebulization chambers 24, one coupled in line with each conduit 20 and 22. Each chamber, made of insulating material, is divided into two compartments 26 and 27. In the first compartment 26 there is provided material having a 20 high surface tension such as metal wool. The returning ink stream enters this compartment and is emitted therefrom in large drops 28 which impinge upon a grounded conductive plate 30 in the second compart ment 27. The conversion of the substantially continu 25 ous ink stream into separated large drops produces an extremely high impedance path to the deflection plate 17. In fact, the impedance between the ink reservoir 4 and the deflection plates increases to a value greater than 100 megohms. After impinging upon grounded 30 conductive plate 30 the fluid passes through conduit 32 to the ink reservoir 4. In order to aid the return flow of ink there is provided the vacuum pump 36 to evacuate air in the reservoir 4. To dry the denebulization chambers for preventing 35 conduction of electrical current from taking place, each chamber 24 includes an aperture 34 through which dry air is forced using any suitable air pump (not shown). The introduction of the dry air dries the cham ber thus decreasing the humidity in the chamber to pre 40 vent electrical current flow. While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art 45 that various changes in form and details may be made therein without departing from the spirit and scope of the invention. What is claimed is: 1. In an electrostatically deflected ink-jet printer pro viding a continuous spray of ink droplets, a method for controlling the interception of said ink droplets with a printing medium comprising the steps of a. providing deflection plates at a non-zero potential for generating an electrostatic field, each plate in 55 cluding a catch-chamber, b. imparting an electric charge to only those droplets which are to intercept said printing medium, c. directing the uncharged droplets to a catch chamber 60 d. breaking the ink received by the catch-chamber into drops to thereby cause the ink to form a high impedance return path to the deflection plates, e. passing each drop over a ground plate whereby the charge acquired by the ink upon passing through the catch-chambers of the deflection plates is neu tralized and f, collecting the neutralized drops of ink in a reser VO.

5 S 2. The method of claim 1 further including the step of disposing said deflection plates at a slight angle with respect to the trajectory of the uncharged droplets. 3. In an electrostatically deflected ink-jet printer of the type including at least one nozzle for ejecting ink 5 toward a printing medium, a charging electrode for se lectively charging ink droplets, and an electrostatic field for deflecting the ink droplets from their trajec tory an amount proportional to the charge thereon, the improvement comprising: a. electrostatic deflection plate having a non-zero po tential thereon for generating said electrostatic field, each of said deflection plates including a catch-chamber for intercepting and collecting sub stantially uncharged droplets, said collected drop lets thereby assuming a charge dependent upon the potential of said plate, b. a reservoir, held at substantially zero potential, for storing said ink to be fed through said at least one nozzle, and - c. a return conduit system for returning the said ink collected by said catch-chambers to said reservoir, said return conduit system comprising, i. a container means, receiving said collected ink in a substantially continuous flow, for emitting said 25 received collected ink in separate and discrete droplets which are relatively large compared to said ink droplets collected by said catch chamber, to thereby result in a relatively large electrical impedance path through said ink in 30 said return conduit system between said catch chamber and said reservoir, and ii. means insulating said container means from said reservoir. 4. The ink-jet printer of claim 3 wherein said deflec- 35 tion plates are disposed at a slight angle with resepect to the trajectory of said substantially uncharged drop lets. 5. The inkjet printer of claim 3 wherein there are a plurality of nozzles, a pair of said deflection plates 40 being associated with each nozzle, each deflection plate being either at a positive or negative potential with respect to ground, said conduit system comprising a first conduit coupled to the catch chambers of said negative potential deflection plates and a second con- 45 duit coupled to the catch chambers of said positive po tential deflection plates, further including first and sec ond of said containers for receiving ink from said con duit system, said first container receiving ink from said 50 O first conduit said second container receiving ink from said second conduit. 6. The inkjet printer of claim 5 wherein at least one plate of each pair of deflection plates associated with a nozzle forms one plate of another pair of deflection plates associated with another nozzle. 7. The ink-jet printer of claim 3 wherein each of said deflection plates is hollow, the hollow portion of the deflection plate forming the catch-chamber. 8. In an electrostatically deflected ink-jet printer of the type including at least one nozzle for ejecting ink toward a printing medium, a charging electrode for se lectively charging ink droplets, and an electrostatic field for deflecting the ink droplets from their trajec tory an amount proportional to the charge thereon, the improvement comprising: a. electrostatic deflection plates having a non-zero potential thereon for generating said electrostatic field, each of said deflection plates being provided with a catch-chamber for intercepting substantially uncharged droplets; b. an ink return conduit system receiving the ink col lected by said catch-chamber, c. a container for receiving ink from said conduit sys tem in a substantially continuous flow, said con tainer comprising a compartment including means for collecting the received ink and emitting it in the form of drops, so that the ink forms a high impe dance return path to said deflection plates, said container further comprising another compartment including a grounded conductive plate for receiv ing said ink drops whereby the charge on the ink drops acquired upon passing through the catch chambers of the deflection plates is neutralized, and d. an ink collecting reservoir for receiving the ink drops after they pass through said container. 9. The ink-jet printer of claim 8 wherein said con tainer is formed from non-conductive materials and wherein said compartment includes high surface ten sion material and said another compartment includes an inlet port for receiving dry air and an outlet port coupled to said reservoir. 10. The inkjet printer of claim 9 wherein said reser voir includes an air outlet port for connection to subat mospheric pressure and an ink outlet port for returning ink to said nozzle. xk k sk sk s 55 60